In the field of a servo system that controls a motor while detecting the rotational angle and the number of rotations of a rotational shaft, it is important to ensure safety. Accordingly, it is important to also ensure the reliability of an encoder that is a constituent element of the servo system by improving a fault detection rate.
For example, a servo system described in each of Patent Literatures 1 to 4 detects an abnormality of an encoder included in the servo system by comparing a plurality of signals relating to a rotational position (such as the number of rotations of a rotational shaft).